The electronic parts used in the CPUs (Central Processing Units) of servers and personal computers, etc. are required to effectively radiate heat generated by the semiconductor elements. To this end, is used the structure that a heat spreader of a material of high thermal conductivity, such as copper or another, is disposed over the semiconductor element with a thermal interface material disposed on the semiconductor element.
The thermal interface material itself is required to be a material of high thermal conductivity and furthermore is required to have the characteristic that the material is able to contact in large areas with minute concavities and convexities of the surfaces of the heat generation source and the heat spreader. At present, as the thermal interface material, PCM (Phase Change Material), indium, etc. are generally used.
However, PCM has good contact to the minute concavities and convexities but has low thermal conductivity (about 1 [W/m·K]˜5 [W/m·K]). For PCM to have effective thermal conductivity, the film thickness must be thin. Between the heat generation source and the heat spreader, gaps take place due to the thermal expansion coefficient difference, and the film thinning is too limited to absorb the concavities and convexities in accordance with the gaps.
The recent large increase of the demand for rare metal has raised the indium price, and substitute materials which are less expensive than indium are expected. In terms of the physical properties, the thermal conductivity of indium (about 50 [W/m·K]) cannot be said high. Materials having higher thermal conductivities are expected so as to effectively radiate the heat generated from semiconductor elements.
In such background, linear structures of carbon atoms represented by carbon nanotubes are noted as a material having higher thermal conductivity than PCM and indium. The carbon nanotubes not only have a very high thermal conductivity (about 1500 [W/m·K]), but also is superior in flexibility and heat resistance. The carbon nanotubes have highly potential as a heat radiation material.
As heat conductive sheets using carbon nanotubes are proposed a heat conductive sheet having carbon nanotubes dispersed in a resin, and a heat conductive sheet having carbon nanotubes grown, oriented on a substrate, which are buried in a resin.
The following are examples of related art of the present invention: Japanese Laid-open Patent Publication No. 2005-150362, Japanese Laid-open Patent Publication No. 2006-147801, and Japanese Laid-open Patent Publication No. 2006-290736.
However, the conventional heat radiation materials using carbon nanotubes have not been able to sufficiently utilize the high thermal conductivity of the carbon nanotubes.